The proposed study is concerned: (1) with the mechanism involved in the transmural passage of blood cells from their extravascular site of formation into the peripheral pool, (2) with the disturbance in this passage in myelogenous leukemia and the possible underlying mechanism of this disturbance, (3) with the entry of malignant leukemic cells originating from a subcutaneous tumor into the circulation, (4) the subsequent organ disposition of the intravascular malignant cells, (5) with the surface characteristics and changes in the surface characteristics of the cellular elements involved in these processes. The methodology includes: transmission electron microscopy, scanning electron microscopy by imaging through secondary electrons and backscattered electrons, the tracing of malignant leukemic cells by 3H-thymidine labelling and liquid scintillation counting and by labelling by diaminoacridines, the use of cell surface reactive agents on the exposed and isolated fixed and living endothelial cell surface of the sinusoidal vessels.